Method for forming solar cell materials from particulars

ABSTRACT

Materials in bulk and film forms are prepared from fine particulate precursors such as single-phase, mixed-metal oxides; multi-phase, mixed-metal particles comprising a metal oxide; multinary metal particles; mixtures of such particles with other particles; and particulate materials intercalated with other materials.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No.DE-FG03-96ER82300 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

CROSS-REFERENCE OF RELATED APPLICATIONS

Not Applicable

BACKGROUND OF THE INVENTION

The present invention generally relates to the formation of materialsuseful for electronic applications, in particular for photovoltaic solarcells and, more specifically, for forming films of such materials fromparticulate precursors.

Manufacturers of electronic devices, in particular photoelectronicdevices and more specifically photovoltaic devices, are increasinglyemploying non-elemental materials such as III-V, II-VI and I-III-VIsemiconductors and alloys, mixtures and layered structures of suchmaterials. The constituent layers in such photoelectronic devices aretypically fabricated using vapor phase deposition processes such asvacuum evaporation, sputtering and chemical vapor deposition. Vaporphase processes are useful for small-scale research and forhigh-precision processing of high-value, small-area devices such asintegrated circuits. Vapor phase processes have yielded solar cells withhigh sunlight-to-electricity conversion efficiencies; but it isdifficult to deposit uniform films on large areas using coincident vaporphase processes in which the constituent elements are co-deposited,hence coincident vapor phase processes are typically costly to scale upto large device sizes with the control and through-put required forcommercial production.

Various researchers have explored sequential vapor phase processes.CuInSe₂ films are typically formed by sequentially depositing solidlayers of Cu and In elemental metals and subsequently reacting the Cu—Incomposite layers with a source of Se to form CuInSe₂ By solid layer wemean a substantially solid layer of material with minimal included voidspace. Multi-step, sequential processes substitute separate sequentialdeposition of constituents in place of co-deposition of constituents,with the intent of mitigating materials interactions that typicallycomplicate co-deposition processes. However, this separation intosequential depositions of different constituents can introduce newcomplications; for example, vapor phase deposited indium tends tode-wet, forming localized islands, and solid layers of Cu—In alloys cansegregate into In-rich and Cu-rich phases when heated, with the resultthat extreme care is required to maintain the desired planar Cu—In layerstructure and lateral composition uniformity during the early stages ofsubsequent reactions to form CuInSe₂.

Various researchers have explored techniques for stabilizing metalprecursor layers such as Cu—In for CuInSe₂ against de-wetting and phasesegregation. For example, thin layers of a chalcogenide metal such astellurium can be deposited on the substrate prior to the deposition ofCu and In in order to form telluride compounds that mitigate indiumde-wetting and phase segregation, and solid layers of binarychalcogenide compounds such as Cu₂Se and In2Se₃ can be deposited andsubsequently interdiffused to form ternary CuInSe₂. Such processes usechalcogenide compounds to stabilize the primary non-chalcogenconstituent metals Cu and In against phase segregation during depositionand subsequent processing.

Other researchers have explored vapor phase processing ofoxide-containing phase-stabilized precursors and mixtures of suchprecursors with elemental metals and non-oxide chalcogenides. Forexample, chalcogenide solid films can be formed by depositing a metaloxide solid film and annealing the oxide film in a gas or vaporcontaining a metal chalcogen such as S, Se, Te or a mixture thereof.Such processes can also utilize layers of single-phase oxideparticulates, such as Cu₂In₂O₅ particles or such as Cu₂O particles mixedwith In₂O₃ particles, and can also utilize solid layers of a mixture ofmetal and oxide, selenide or sulfide compound constituents. Whilemitigating certain complications of sequential processes, thesephase-stabilized precursor improvements leave unresolved the inherentcomplexities of achieving the constituent layer uniformity necessary toachieve high-quality semiconductor films using vapor phase processes.

Various researchers have explored alternatives to vapor phase processesfor fabricating semiconductor materials for various photoelectronicapplications. Electrodeposition can be used to sequentially depositsolid layers of constituent metals such as Cu and In that aresubsequently reacted with chalcogenide metals such as Se to formcompound semiconductor films. The chalcogenide metal can be embedded inthe electroplated solid metal layers by adding Se particles to theelectroplating bath so as to incorporate Se particles into one or moreof the metallic layers. Additional Se can be added by screen printing asolid Se layer on the Cu—In—Se precursor layers, or by annealing theelectrodeposited layers in Se vapor. While avoiding some of thedisadvantages of vapor phase processes, such electrodeposition processesare plagued with the challenges of uniform high-rate electrodepositionon large-area substrates and introduce electrodeposition-specificcomplications such as metal-contaminated waste treatment, recovery anddisposal.

Other researchers have explored alternatives to both vapor phase andelectrodeposition processes. For example, spray pyrolysis techniques canbe used to deposit metal oxide solid layers, and the oxide layers cansubsequently be annealed in chalcogenide metal vapor to convert theoxide layers to chalcogenide films. Spray pyrolysis is convenient fordepositing multi-component oxides on large areas; but the materials useefficiencies of spray pyrolysis processes are generally low, hencemanufacturing materials costs are generally high.

Alternatively, one can form precursor layers by screen printing a pasteof particles or by painting a substrate with a slurry of particles. Forexample, one can form a Cu—In—Se powder, prepare a paste from thepowder, screen print layers of the paste, and anneal the layers to formCuInSe₂ films. Cu—In—Se powders prepared by ball milling or grindingreportedly yield median particle diameters of 1.5 μm and larger. Medianpowder particle diameter determines minimum pinhole-free CuInSe₂ layerthickness; particle diameters of 1.5-2 μm typically limit CuInSe₂ filmthickness to 5 μm or greater. Such film thickness are a factor of 5-10thicker than necessary to absorb incident sunlight, and result in highmanufacturing materials costs. Researchers preparing CuInSe₂ films byscreen printing and sintering CuInSe₂-based pastes report takingparticular measures to avoid the formation of indium oxides deleteriousto the CuInSe₂ film properties. Screen-printed films are typically muchthicker than required to absorb sunlight sufficiently. Screen printingand related film formation processes are unlikely to be economic unlesseffective strategies for forming powders with smaller particle diametersand for processing the powders to achieve good film quality aredeveloped.

Various researchers have investigated small particles with medianparticle diameters of 100 nm and less as a pathway to preparingthin-film materials. Nanoparticles of a wide range of oxides (e.g. ZnO,SnO₂, WO₃, etc.) and chalcogenides (e.g. CdS, CdTe, etc.) have beenreported, and thin films have been formed from nanoparticles by avariety of techniques. Such small particles can be deposited asparticulate layers by a variety of processes including, for example,electrophoresis of colloidal suspensions and slurry spraying. CuInSe₂films have been prepared by spraying slurries of mixtures ofsingle-phase, binary selenide nanoparticles such as Cu_(x)Se andIn_(x)Se, but film quality and device performance are poor due toinsufficient interparticle diffusion. This implies that small medianchalcogenide particle diameters alone do not provide improvedparticle-based thin film properties.

The use of an effective flux is known to be particularly important forpromoting particle coalescence and grain growth in particle-based thinfilms. CdCl₂ works well as a flux with Cd-based chalcogenide materialssuch as CdTe and Cd(Se,Te). A comparable flux has not been reported forCuInSe₂ and related alloys. Se, CuCl, InCl₃ and Cu-Se compounds havebeen evaluated as fluxes for screen-printed CuInSe₂ layers, andnon-negligible fluxing reportedly occurs only at relatively hightemperatures in Cu-rich material when a liquid Cu₂Se is present. Thuseffective fluxing of CuInSe₂ is possible only under conditions thatresult in Cu-rich CuInSe₂ films unsuitable for solar cells, or incomplex multi-step processes such as continuous co-evaporation in whichthe growing CuInSe₂ film is temporarily made Cu-rich to effect transientfluxing via liquid Cu₂Se before differentially adding In and Se toachieve an In-rich final film composition. Results are also poor whenpaste mixtures of Cu and In elemental powders are screen printed andovercoated with screen-printed Se powder paste.

Summarizing the prior art, coincident vapor phase processes aredifficult to control on large areas; sequential vapor phase processessimplify some of the complexities, but composition uniformity andprecursor phase segregation can be severe problems. Metal oxides areuseful as phase-stabilized precursors, but uniform, high-rate, vaporphase deposition of oxide solid films is difficult, and spray pyrolysisof oxide solid films is inefficient due to poor materials useefficiencies. Particulate-based processes such as screen printing canhave high materials efficiencies, but such processes generally work wellonly when efficient fluxing processes are available. A clear need existsfor phase-stabilized precursors that can be easily converted tothin-film, photovoltaic materials. A need also exists for an easilycontrolled process for forming such precursors. It would be especiallyadvantageous to provide a process using particulate precursors tofabricate thin films with well-controlled compositions on large areas,as well as materials and processing techniques for creating highquality, thin film products without contamination from residual fluxmaterials.

BRIEF SUMMARY OF THE INVENTION

The present invention provides unique methods for makingphase-stabilized precursors in the form of fine particles, such assub-micron multinary metal particles, and multi-phase mixed-metalparticles comprising at least one metal oxide.

The invention further provides methods of using spraying and coatingtechniques to deposit thin, close-packed layers of multinary metaland/or mixed-metal metal oxide particles. In one embodiment slurryspraying is used to deposit layers of single-phase, mixed-metal oxideparticulates from aqueous slurries sprayed in air on a heated substrate.

In one aspect of the invention, compound materials are formed usingprecursors comprising multi-phase particles comprising a metal oxidephase. In one embodiment, particulate precursors are deposited as layerson suitable substrates by efficient processes such as slurry spraying.The precursor layers are then converted to useful films by reactingprecursor layer components together so as to cause interdiffusion,and/or by reacting the precursor layers with other reactant materials,such as overcoated layers, liquids, vapors or gases, causing ionexchange and interdiffusion. Of particular advantage are multi-phaseparticles in which each particle contains more than one compositionalphase; multi-phase particulate precursors comprising metal oxides yieldsuperior final film characteristics relative to single-phase metal oxideparticulate precursors or precursors made up exclusively of non-oxidecompound particles.

In another aspect of the invention, the precursors are multi-phaseparticles comprising both a metal oxide phase and a non-oxide phase. Thepresence of a non-oxide phase provides advantageous reaction pathwaysfor converting particulate precursor materials to high-quality finalmaterials by, for example, facilitating transient fluxing byintermediate phases during the conversion process. Of particularadvantage are multi-phase particles comprising a metal oxide and a metalphase or binary compound phase in which the metal or compound phasefacilitates fluxing and densification by forming liquid phases thatfacilitate transient fluxing bet ween precursor particles during theconversion of the precursor to the final material. The differingconversion rates of the multiple phases of the precursor particlesprovides a pathway for transient fluxing due to a localized relativeabundance of one or more constituents in a precursor material that hasan overall deficiency of those constituents.

In another aspect of the invention, multinary metallic particles andmixtures of multinary metallic particles are used as precursors tofurther augment the advantages of particles comprising non-oxide phases,and to further simplify the conversion of precursor materials todesirable final materials. Of particular advantage are multinary,metallic particles that allow films of mixed-metal compounds to beprepared without the phase segregation problems typically associatedwith annealing solid mixed-metal layers.

In another aspect of the invention, there are provided methods forforming porous precursor layers intercalated with other materials,thereby further simplifying the conversion reactions of precursor layersto final films and further improving the density and electronic qualityof the final films. Intercalation reduces the void space within aparticulate-based layer and facilitates solid state reactions to formdense, coherent films. Intercalation provides pathways to facilitatetransient localized fluxing in precursor layers that, throughinterdiffusion, reach efficient final film composition.

The present invention provides methods for making multi-phasemixed-metal particles comprising at least one metal oxide, and methodsfor making multinary metal particles. The invention teaches the use ofslurry spraying, spray printing, spin coating and meniscus coating todeposit layers of particulate materials. The invention exploits theheretofore unrecognized advantages of using multi-phase particles asprecursor materials for forming many desirable materials. In particular,the invention provides a route to thin-film materials using uniqueprecursors such as metal oxide phases, non-oxide phases, and metallicphases. The invention further teaches that the utility of all kinds ofparticulate precursors can be augmented by intercalating the particleswith other useful materials so as to facilitate low-temperature,solid-state ion exchange and densification. The full spectrum of uniqueadvantages of this invention are more completely evident in theembodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects and details of the invention will become apparent whenconsidered in conjunction with the attached drawings, wherein:

FIG. 1 is a schematic representation of the formation sequence of bulkmaterials in accordance with one method of the present invention; and

FIGS. 2 and 3 are schematic representations of the formation sequencesof films of materials in accordance with alternative methods of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the text, the following definitions are used:

materials—bulk forms, such as free-standing solids or particles, or filmforms, such as coatings, layers, or films on a substrate.

compound—comprising two or more chemically-bonded elements.

particulate—solid-state, granular pieces; not liquid droplets.

precursor

materials—source materials in intermediate bulk or film form that can besubsequently converted to a final desired material.

phase—a constituent region of specific composition.

multi-phase—containing more than one phase.

mixed-metal—containing two or more metals.

metal oxide—a compound comprising oxygen and at least one metal.

non-oxide—a chemical moiety substantially devoid of oxygen; or, achemical moiety in which oxygen is present only as part of amulti-element ion.

multinary—containing two or more elements.

metallic—consisting of elemental metals, metalloids, or alloys ofelemental metals and/or metalloids, such that the resulting phases(s)is(are) not multinary compound(s). “Metallic” includes, for example,homogeneous particles of a single-phase alloy of Cu and In; multi-phaseparticles containing for example, separate Cu, In and Cu—In alloyphases; and multi-phase particles containing Cu, In, and Se as metalsand/or alloys. Excluded are precursors containing stable compounds, suchas Cu_(x)Se, In_(x)Se and Cu_(x)In_(y)Se, other than de minimisinterdiffused layers that may exist, for example, at the interfacebetween a Cu—In metal phase and an adjacent Se metal phase.

The present invention provides processes for forming unique precursorparticulate materials, and compound materials made from such precursormaterials. The preferred embodiments of this invention are clearlydemonstrated in the case of the preparation and processing of CuInSe₂and related alloys, and relevant illustrative examples are for the mostpart given below relative to CuInSe₂ , its alloys with Ga and S, anddesirable additives such as Na. The considerable advantages of thisinvention are, however, not limited to these materials; rather, theimprovements embodied in this invention are useful for a wide range ofbulk and film materials, including but not limited to I-III-VI, II-VI,III-VI, III-V, and II-IV-V compounds and various alloys and dopants ofthese.

In a first aspect of the invention, a method is provided for making fineparticles, such as sub-micron, mixed-metal particles comprising a metaloxide phase or both a metal oxide phase and a non-oxide phase. Thisfirst aspect also provides a method for making fine metallic particles,such as sub-micron, multinary, metallic particles.

Particulate materials can be prepared by numerous methods, includinglaser pyrolysis, atmospheric arc evaporation, solution precipitation,chemical vapor reactions, aerosol pyrolysis, vapor condensation, laserablation and other such methods as known in the art. Aerosol pyrolysisis particularly useful for low-cost production of mixed-metalparticulates comprising metal oxides, and multinary metallicparticulates, in particular, substantially solid, substantiallyspherical particulates. Aerosol pyrolysis is also useful for addingadditional desirable constituents such as dopants and alloying elements,such as Ga in CuInSe₂, and desirable additives, such as Na, in CuInSe₂and its alloys.

Aerosol pyrolysis processes can be used to form very fine sized powdersof metal oxide materials by atomizing metal salt solutions into finemist droplets and pyrolyzing the droplets to particles in a suitableenvironment. Metal oxide powders can be subsequently reduced to metallicpowders by annealing in suitably reducing circumstances, such as in anatmosphere comprising H₂ gas. Metallic powders can be directly producedby aerosol pyrolysis by proper selection of solution composition andreducing reaction environment. Hybrid aerosol pyrolysis processes inwhich particles suspended in droplets of a salt solution are pyrolyzedto form composite particles are particularly suitable for makingmulti-phase, composite particles where an oxide phase is present with anon-oxide phase such as a metal or non-oxide chalcogenide phase, andwhere one or more phases substantially envelop one or more other phases.

In one embodiment, this first aspect of the invention comprisesatomizing source materials and introducing the atomized source materialsinto an environment where they react to form multi-phase, mixed-metalparticles. Suitable source materials include, without limitation,solutions for spray pyrolysis as known in the art. A particularly usefulsolution is an aqueous solution of metal salts, such as nitrates,.sulfates and halides, more preferably aqueous solutions of two or moremetal nitrates in concentrations of about 0.01-1000 milliMolar, morepreferably about 0.1-10 milli-Molar. Other source materials includemixed-metal solids, such as alloy wires or powders usable as sourcematerials for flame spraying, plasma gun atomization and other methodsthat create molten metal droplets or metal vapors. Source materialsaccording to this first embodiment are mixed-metal materials comprisingtwo or more metals in atomic ratios necessary to yield the desiredcomposition of the final particulate materials.

Source materials can be atomized by various means, including forexample, ultrasonic nebulizers, electrostatic nebulizers, pneumaticnebulizers, spinning wheel atomizers, impact atomizers and other suchmethods as known in the art. Ultrasonic nebulizers operating atfrequencies above 1 MHz are particularly advantageous for producingsource solution aerosols with 0.35-10 micron diameter droplets.

Suitable environments for reacting source materials to form multi-phasemixed-metal particles according to this first embodiment includeenvironments in which a source of energy is present to drive thereaction of source materials to final particulate materials. Suchenergetic environments include, for example, near-atmospheric-pressurefurnaces, vacuum furnaces, microwave furnaces and other suchenvironments as known in the art and capable of heating a materialtherein. A suitable environment according to this first embodiment isone capable of heating the atomized source materials to a temperatureand for a time sufficient for the source materials to react to form thedesired final multinary material. Alternatively, if the source materialis atomized in such a way as to be inherently highly energetic, such asby being atomized as a molten metal or metal vapor, then the reactionenvironment need not add any additional energy to the atomized sourcematerial. Suitable reaction environments according to this firstembodiment include gaseous environments where the ambient pressures areat absolute pressures above, below, or near atmospheric pressure, suchas the environment produced in a high-pressure autoclave, a vacuumfurnace, and a quartz tube furnace, respectively. Anear-atmospheric-pressure, flowing-gas furnace operating at 350°-1050°C. and operated so as to allow 3 or more seconds residence time ofsources and reactants in the hot zone is particularly advantageous as areaction environment for atomized aqueous source solutions.

Suitable ambient gases include oxidizing gases, such as oxygen and air;substantially inert gases such as nitrogen and noble gases; and reducinggases such as hydrogen, hydrogen mixed with inert gases (for example,nitrogen), and hydride gases such as H₂Se, H₂S, etc. Oxidizing ambientgases are particularly advantageous for making oxide final materials;for example, oxygen is advantageous in making single-phase Cu₂In₂O₅particles. Inert ambient gases are particularly advantageous for makingmulti-phase materials comprising a metal oxide; for example, nitrogen isadvantageously employed for reacting aqueous nitrate solutions to makemulti-phase Cu₂O—In₂O₃ particles in which each particle has localizedregions of Cu₂O and In₂O₃. Reducing ambient gases are particularlyadvantageous for making multi-phase materials comprising a metal phaseor comprising metal oxide phase and a non-oxide phase; for example, 10vol % H₂ in N₂ is particularly advantageous for reacting aqueous nitratesolutions to make Cu—In₂O₃ metal-metal oxide particulate sources inwhich each particle comprises a metallic Cu phase and an In₂O₃ oxidephase.

Source materials, ambient gas, and reaction energetics must becontrolled to achieve multi-phase particles. For example, single-phaseparticles of CuO, Cu₂O and Cu can be produced by aerosol pyrolysis ofcopper nitrate solutions by suitably selecting furnace ambients andoperating temperatures. Thus, pyrolysis in nitrogen at 1200° C. yieldsCu₂O; pyrolysis in nitrogen at 700° C. yields CuO; and pyrolysis ofcomparable source solutions in 7% H₂ in N₂ at 700° C. yields metallicCu. By properly choosing source materials, ambient gas and reactionenergetics, one can form multi-phase, mixed-metal materials by selectinga pyrolysis process that causes the metal reactants to reach specificdegrees of oxidation. For example, a dilute aqueous solution of equalparts copper nitrate and indium nitrate will form single-phase Cu₂In₂O₅when pyrolyzed at about 900° C. in O₂, multi-phase Cu₂O—In₂O₃ whenpyrolyzed at about 900° C. in N₂ and multi-phase Cu—In₂O₃ when pyrolyzedat about 500° C. in 10 vol % H₂ in N₂. Similar results are possibleusing other source materials such as molten metal droplets and metalvapors. Different mixed metal groupings such as Zn—Al, In—Sn, etc. willyield single-phase oxides, multi-phase oxides, metal—metal oxidecomposites, or mixed-metal metallic particles depending on the relativereaction rates and oxidation rates of different metal-containingreactants in the temperature, time and ambient gas characteristics ofthe reaction environment.

The advantages of this first aspect of the invention are realizable on awide variety of materials. For example, particulate materials that canbe advantageously processed this way include oxide, chalcogenide, metaland hybrid materials such as Cu₂In₂O₅, Cu—In—Se, Cu—In and Cu₂Se—In₂O₃useful as particulate precursors for forming CuInSe₂ and its alloys.Other particulate materials can also be processed according to thisembodiment; for example, In_(x)O—Sn_(x)O materials for forming indiumtin oxide (ITO), Zn—Al materials for forming Al-doped ZnO, ZnO—CdOmaterials for forming Zn_(x)Cd_(y)Te, and Zn_(x)P_(y)O—ZnO materials forforming Zn_(x)P. These advantages of this first aspect is particularlyevident in Group IIB mixed-metal materials such as ZnO—CdO, Groups IBand IIIB mixed-metal materials such as Cu₂In₂O₅, and Groups IIIB and IVBmixed-metal materials such as ITO.

A second aspect of the invention is the use of slurry spraying andmeniscus coating methods to deposit particulate layers as precursors forforming thin films.

Particulate precursor layers provide key advantages over solid precursorlayers. Solid precursor layers, such as layers of Cu—In and other metalalloys, are subject to inhomogeneities due to phase segregation, e.g.segregation into liquid In and solid Cu-rich Cu_(x)In_(y)phases, attemperatures typically used to convert Cu—In alloys to CuInSe₂ usingsuch conversion pathways as annealing in Se vapor. In contrast,particulate precursors limit the impact of phase segregation byphysically constraining a particle's constituents to the dimensions ofthe particle.

Mixed-metal particles can also improve final composition uniformitycontrol by decoupling sequential constituent addition uniformity fromfinal materials uniformity. For example, in contrast to sputtered orvacuum evaporated Cu and In precursor layers typically used forpreparing CuInSe₂ films, which layers generally exhibit spatialvariations in Cu/In composition, particulate precursors uniformlycontaining Cu and In in the desired atomic ratio can preserve thedesired CuInSe₂ composition, even when CuInSe₂ film thickness varies dueto precursor layer deposition inhomogeneities.

Some of the advantages of particulate, mixed-metal precursors, notably,ease of thin film deposition, can be achieved by using particulates thatare substantially composed of the desired final material. However,particles composed of the desired final material often yield inferiorfilm quality. For example, CuInSe₂ particles can be used to form aCuInSe₂ solid or film, but, without effective fluxing (such as by CdCl₂in CdTe) or special materials formation pathways (such as lowereffective melting temperatures for nanoparticles of very small diameters(e.g., 20 nanometers or less)), particulate precursors of the desiredfinal composition, or mixtures of stable compounds of the constituentelements, such as mixtures of Cu₂Se and In₂Se₃, generally result infinal materials with poor cohesion, low density and inferior electronicproperties. In contrast, precursors that undergo chemical and/orstructural conversions, such as metallic Cu—In precursors subsequentlyconverted to CuInSe_(with an accompanying 2 to 3 times volumeexpansion), are more likely to yield cohesive, dense CuInSe₂ materialsthan would comparably processed binary or ternary selenide precursors.

One can also mitigate precursor phase segregation and provide forcohesion and densification by using phase-stabilized precursor materialssuch as oxides and other compounds. Solid metal oxide precursors such asCu₂O, In₂O₃, Cu₂In₂O₅ and alloys of these with other compounds areuseful in that such oxide phases are more stable against phasesegregation at temperatures of 150-700° C. typically used to convertprecursor materials to chalcogenide materials; but solid metal oxidefilms are difficult to deposit at high rates on large areas. Particulateprecursors comprising a metal oxide phase are particularly advantageousin comprising phase-stabilized precursors and in providing for easy andefficient deposition in film form by such low-cost techniques asspraying and printing.

Particulate precursor layers can be deposited by numerous methods. Forexample, screen printing is widely used to deposit Ag-paste grid lineson silicon solar cells, and screen printing is used to deposit CdTepastes to fabricate large-area, thin-film solar cells. Stencil printing,writing and nozzle printing have been used to deposit narrow grid linesof particle-based pastes, and writing and nozzle printing techniqueshave been used for depositing CdTe layers on large-area substrates.While screen printing, stencil printing, writing and nozzle printingtechniques are usable for applications where layer thickness is notcritical, such techniques are typically limited to minimum layerthicknesses of 5 to 10 microns. These minimum practical layerthicknesses are substantially larger than the 1 to 2 micron thicknessestypically optimal for thin-film photovoltaics applications, thus thesetechniques are relatively materials-intensive and costly.

Slurry spraying and meniscus coating methods are advantageous inproviding fast, uniform deposition of very thin layers, and the use ofsuch methods to form precursor layers for thin-film photovoltaic filmsfor solar cells is the second aspect of the invention. Slurry sprayingmethods, such as pneumatic spraying with a pressurized gas nozzle,hydraulic spraying with a pressurized slurry expelled from through anorifice, ultrasonic spraying with rapidly vibrating atomization surfaceand electrostatic spraying with a high-voltage electric field to directthe atomized slurry to the part, and meniscus coating methods, such asdip coating in a bath or Langmuir trough, spin coating on a rotatingstage, waterfall coating in which the substrate travels through acascading sheet of slurry, and in-line meniscus coating in which thesubstrate contacts a slurry pool, are useful for depositing thin,uniform layers of particulate materials on large-area substrates. Suchmethods are in general not limited by the particle delivery fixturing,such as screen and emulsion thicknesses as in the case of screenprinting.

Slurry spraying is particularly advantageous in providing acost-effective method of depositing thin films on large areas with lowcapital equipment and indirect materials costs. For example, uniformparticle layers of less than 1 micron thickness can be deposited byslurry spraying particles with mean sizes of 0.25 micron or less, andthe slurry spray can be rastered to uniformly coat large areas. Slurryspraying can achieve a materials use efficiency of more than 90%.

Slurry spraying can accommodate a wide variety of slurry particles,solvents, additives and spraying conditions to achieve advantageouseffects. For example, slurry spraying can accommodate a wide variety ofparticle types, such as single-phase Cu₂In₂O₅ oxides, multi-phaseCu₂O—In₂O₃ oxides, multi-phase Cu—In₂O₃ materials, and metallic Cu—Inmaterials.

Slurry spraying can accommodate a wide variety of solvents, such aswater, alcohols, glycols, and mixtures of such solvents. Water isparticularly advantageous as a safe and simple slurry solvent into whichmany types of metal and metal oxide particles will adequately disperse.

Slurry spraying can accommodate a wide variety of slurry additives suchas dispersants, thickeners, binders and surfactants to optimize layerproperties. A 1 to 5 volume percent of an alcohol such as ethanol is aconvenient dispersion aid for aqueous slurries containing metal and/ormetal oxide particles, as are various commercial dispersants such asRohm and Haas Duramax 3002, 3007 and 3019.

Slurry spraying can accommodate a wide degree of solids loading anddeposition rates. A solids loading of 0.1 to 200 g/L is preferable forslurry spraying of a water-based slurry of sub-micron metal and/or metaloxide particles. A solids loading of 0.5 to 25 g/L is more preferableand of 5 to 10 g/L is most preferable for 0.1 micron mixed-metalparticles comprising a metal oxide.

Slurry spraying can be effected in air or in a controlled environmentsuch as an inert or reducing atmosphere that will mitigate particleoxidation.

Slurry spraying can be effected by a variety of means, including forexample by entraining the slurry in a gas flow such as in a pneumaticair brush, by atomizing the slurry from a surface such as in anultrasonic sprayer, or by expelling the pressurized slurry through ahydraulic nozzle. Pneumatic slurry spraying is particularly advantageousfor minimizing spray equipment costs and complexity. Ultrasonic slurryspraying is particularly advantageous for minimizing substrate coolingand slurry losses due to gas flow. Electrostatic slurry spraying isparticularly advantageous for maximizing materials use efficiency.

The characteristics of slurry-sprayed layers can be affected bysubstrate surface temperature. In general it is best if the substratesurface temperature is high enough to vaporize the slurry solventrapidly enough to avoid slurry pooling and low enough to provide forsolvent-facilitated lateral movement of particles on the growing layersurface. Excessively low surface temperatures allows solvent build-upand results in macroscopically non-uniform layers. Excessively highsurface temperatures results in the solvent evaporating in flight or tooquickly on arriving at the substrate surface and yields microscopicclumping of particles and poor microscopic particle coverage. A heatersurface temperature of about 100°-150° C. is advantageous for aqueousslurries.

The advantages of this second aspect of the invention are realizable ona wide variety of materials. One embodiment of this second aspect isspraying an aqueous slurry of single-phase Cu₂In₂O₅ particles to formphase-stabilized precursor layers for subsequent conversion intoCuInSe₂. Just as Cu₂In₂O₅ particulate layers can be deposited by slurryspraying as precursor layers to form CuInSe₂, ZnO—CdO particulate layerscan be spray-deposited as precursor layers to form ZnCdTe₂, and In—Sn—Oparticulate layers can be spray-deposited as precursor layers to formITO.

A third aspect of the invention comprises the formation of bulk or filmmaterials from multi-phase, mixed-metal, particulate precursor materialscomprising a metal oxide phase.

Referring to FIG. 1, this third aspect of the invention is schematicallydepicted as comprising the step of converting one or more particulateprecursor materials 1 to desired bulk materials 2, wherein saidprecursor materials comprise multi-phase, mixed-metal particlescomprising a metal oxide.

Referring to FIG. 2, this third aspect of the invention is furtherdepicted as also comprising the steps of (a) depositing one or moreprecursor layers 3 on a substrate 4 of one or more particulate precursormaterials 1, where the precursor materials comprise multi-phase,mixed-metal particles comprising at least one metal oxide, and (b)converting the precursor layers to one or more desired films 5.

Multi-phase, mixed-metal, particulate precursors comprising a metaloxide phase provide key advantages over solid precursors andsingle-phase particulate precursors. Fine-grained, single-phase,mixed-metal oxide particulate precursor layers capture many of theadvantages of particulate precursor layers comprising oxides, butsingle-phase particulate precursor layers such as Cu₂In₂O₅ converted tofilms of final materials such as CuInSe₂ by such processes asvapor-phase, gas-phase or solid state reactions with one or morechalcogen-containing materials (such as Se vapor, H₂Se or a Se coating)often yield final materials with low densities and poor electronicqualities, presumably as a result of the precursor particles expandingin diameter upon conversion without coalescing or conforming to fill thevoids present prior to conversion. The use, instead, of mixtures ofconstituent single-phase oxide particulates such as Cu₂O and In₂O₃ canyield somewhat better film cohesion and density due, presumably, to thediffering conversion rates and intermediary properties of the differentparticulate compositions. However, the physical segregation ofconstituents in separate particles generally requires that thick filmsbe processed for long times at high temperatures to achieve suitablefinal film composition uniformity. Thick films waste materials andincrease costs, and long, high-temperature processing severely limitsthe spectrum of suitable substrates and workable solar cell structures.

Multi-phase, mixed-metal, particulate precursors comprising a metaloxide simultaneously provide the advantages of particles,phase-stabilized oxides and multi-material reactions. Thus, two-phase,Cu₂O—In₂O₃ particles in which each particle has localized regions ofCu₂O and In₂O₃ can yield thin, CuInSe₂ films comparable in quality tofilms produced from single-phase Cu₂In₂O₅ particles or thicker filmsproduced from mixtures of single-phase binary oxide particles processedfor longer times at higher temperatures. Similarly, while bothmulti-phase Cu₂O—In₂O₃ and single-phase Cu₂In₂O₅ particulate precursorlayers can yield homogeneous CuInSe₂ films when reacted with seleniumvapor under suitable conditions, CuInSe₂ films prepared from multi-phaseprecursors exhibit larger grains and higher densities than do CuInSe₂films prepared from single-phase precursors using comparableselenization conditions.

The multiple phases of a multi-phase precursor particle can besegregated in any fashion within a particle. For example, phases can bemacroscopically segregated within a two-phase particle, as in suchidealized cases as where two phases constitute separate hemispheres of atwo-phase spherical particle or where one phase forms a concentric shellaround a spherical subparticle of the other phase.

Embodiments where one or more phases substantially envelop other phasesare particularly advantageous when the outer phases exhibit differingconversion rates and intermediary properties that facilitate improvedfinal material properties. For example, given that a liquid Cu_(x)Sephase within a CuInSe₂ film can act as a flux to promote grain growth, aprecursor comprising Cu₂O—In₂O₃ multi-phase particles of Cu₂O around asubparticle core of In₂O₃ should yield the transitory existence ofCu_(x)Se acting to promote particulate layer cohesion and densificationduring conversion to the selenide, even for Cu₂O—In₂O₃ multi-phaseparticles that are In-rich overall.

The advantages of multi-phase, particulate precursors containing a metaloxide can also to be realized with mixtures of such precursors withother particulate materials. For example, multi-phase, particulateprecursors comprising multiple oxide phases such as Cu₂O—In₂O₃ can bemixed with other particulates, such as Cu, Cu_(x)O, Cu_(x)Se and Separticulates, to facilitate densification and grain growth throughfluxing. Such mixtures of particulate precursors can be effected bymechanical mixing of precursor materials before deposition, byco-depositing the multiple precursor materials, or by sequentiallydepositing the different precursor materials.

The advantages of multi-phase particulate precursors containing a metaloxide can also be realized by combining such precursors with otherprecursor materials. For example, layers of multi-phase, particulateprecursors comprising metal oxide phases, such as In-rich Cu₂O—In₂O₃,can be combined with solid layers of Cu or other metal deposited byphysical vapor deposition, solution deposition, and other techniques asknown in the art; such composite precursors combining multi-phase,particulate layers and other layers facilitate advantageous reactionpathways, such as Cu_(x)Se fluxing.

The advantages of this third aspect are realizable on a wide variety ofmaterials. Compound materials that can be processed this way includephotovoltaic materials such as CuInSe₂ and its alloys with Ga and S,transparent conductors such as indium tin oxide (ITO), ZnO and ZnO:Al,optoelectronic materials such as Zn_(x)Cd_(y)Te and Hg_(x)Cd_(y)Te, andsemiconductor materials such as Al_(x)Ga_(y)As and Zn_(x)P.

For example, just as Cu₂O—In₂O₃ multi-phase, particulate precursors areadvantageous for forming CuInSe₂, so, too, will In₂O₃—SnO₂, ZnO—CdO andAlO_(y)—GaO_(x) particulate precursors be advantageous for forming ITO,Zn_(x)Cd_(y)Te and Al_(x)Ga_(y)As, respectively. Similarly, just asCu₂O-coated In₂O₃ and In₂O₃-coated Cu₂ particles would be advantageousfor forming CuInSe₂, ZnO-coated CdO particles would be advantageous forforming Zn_(x)Cd_(y)Te. And just as mixtures of Cu₂O—In₂O₃ and Cuparticles may yield benefits in forming CuInSe₂, mixtures of ZnO—CdO andZn particles may yield benefits in forming Zn_(x)Cd_(y)Te.

A fourth aspect of the invention is the formation of bulk or filmmaterials from multi-phase, mixed-metal, particulate precursor materialscomprising both a metal oxide phase and a non-oxide phase.

Referring again to FIG. 1, a first step of this aspect of the inventionis schematically depicted as the converting of one or more particulateprecursor materials 1 to desired bulk materials 2, wherein the precursormaterials comprise multi-phase, mixed-metal particles comprising a metaloxide phase and a non-oxide phase.

Referring again to FIG. 2, this fourth aspect of the invention isschematically depicted as the steps of (a) depositing one or moreprecursor layers 3 on a substrate 4 of one or more particulate precursormaterials 1, where the precursor materials comprise multi-phase,mixed-metal particles comprising both metal oxide and non-oxide phases,and (b) converting the precursor layers to one or more desired films 5.

Multi-phase, mixed-metal, particulate precursors containing both oxideand metal phases provide particular advantages. For example, Cu—In₂O₃particulate precursors in which the particles comprise a metallic Cuphase and an In₂O₃ oxide phase yield differential grain growth,presumably due to Cu_(x)Se fluxing, given the differential rate ofselenization of metallic copper versus the ion exchange conversion ofindium oxide to indium selenide, and the solid state diffusion of thebinary compounds to form the ternary selenide. As another representativeexample, Cu₂In₂O₅—Se multi-phase, particulate precursors incorporateadditional constituents into the precursor, simplifying subsequentconversion to the desired final material. Again, separate phases can besegregated in any fashion, with the substantially enveloped structuresproviding particular advantages.

Multi-phase, mixed-metal, particulate precursors containing both oxideand non-oxide compound phases can also provide other significantadvantages. For example, Cu_(x)Se—In₂O₃ particulate precursors provide adirect pathway to Cu_(x)Se fluxing. Again, separate phases can besegregated in any fashion, with the substantially enveloped structuresproviding particular advantages.

As with precursors comprising multi-phase, mixed-metal, particulatematerials comprising only metal oxide phases, additional advantagesaccrue with mixtures of particles and with composites with otherprecursor materials. For example, In-rich Cu—In₂O₃ particles mixed withCu and/or Se particles, and layers of Cu—In₂O₃ particles combined withlayers of Se, facilitate Cu_(x)Se fluxing and/or simple solid statereactions.

The advantages of this fourth aspect of the invention are realizable ona wide variety of materials. For example, just as Cu—In₂O₃ multi-phase,particulate precursors are advantageous for forming CuInSe₂, so, willtoo, In—SnO₂, Zn—CdO and Zn—Al₂O₃ particulate precursors be usedadvantageously to form ITO, Zn_(x)Cd_(y)Te and Al-doped ZnO,respectively. Similarly, just as Cu_(x)Se—In₂O₃ particles areadvantageous for forming CuInSe₂, ZnO—CdS and ZnO—CdSe particles will beadvantageous for forming Zn_(x)Cd_(y)S and Zn_(x)Cd_(y)Se, respectively.Just as Cu—coated In₂O₃ particles would be advantageous for formingCuInSe₂, Zn-coated CdO particles would be advantageous for formingZn_(x)Cd_(y)Te. And just as mixtures of Cu—In₂O₃ and Cu particles willyield benefits in forming CuInSe₂, mixtures of Zn—Al₂O₃ and Zn particleswill yield benefits in forming Al-doped ZnO and mixtures of Zn—P₂O₅ andZn particles will yield benefits in forming Zn_(x)P.

A fifth aspect of the invention is the formation of bulk and filmmaterials from multinary, particulate, metallic precursor materials andmixtures of such materials.

Referring again to FIG. 1, this aspect of the invention comprises,initially, converting one or more precursor materials 1 to desired bulkmaterials 2, wherein the precursor materials are multinary, particulate,metallic precursor materials or mixtures of such materials.

Referring again to FIG. 2, this aspect of the invention comprises thesteps of (a) depositing one or more precursor layers 3 on a substrate 4of one or more particulate precursor materials 1, where the precursormaterials are multinary, particulate, metallic precursor materials ormixtures of such materials, and (b) converting the precursor layers toone or more desired films 5.

Multinary, particulate, metal precursor materials provide key advantagesover solid metal precursors and solid layered metal precursors.Multinary particles improve final composition uniformity control bydecoupling sequential constituent uniformity from final materialsuniformity. Metal precursor particles are more likely to yield cohesive,dense final materials than would comparably processed particles of thefinal desired material. By physically constraining a particle'sconstituents to the dimensions of the particle, particulate precursorslimit the impacts of phase segregation common in solid metal precursors.

Multinary, particulate, metal precursor materials can be single-phase ormulti-phase materials. For example, Group I and III metals can formsingle-phase alloys, such as Cu₁₁In₉, and multi-phase materials, such asregions of pure In within a Cu-rich, Cu—In alloy bulk mass. The multiplephases of a multi-phase, metal precursor can themselves be elementalmetals or metal alloys; multi-phase materials include, for example,elemental phases of Cu, In and Se, and alloy phases of Cu—In, Cu—Ga,etc.

The multiple phases of multi-phase, multinary, particulate metalprecursor materials can be segregated in any fashion within a particle,including, for example, macroscopically segregated within a particle oras a substantially enveloping coating of one or more phases around innersubparticles of other phases. A particularly preferred case is where oneor more phases substantially envelop other phases, for example, Cu on anIn subparticle.

Precursor layers of multinary, metal particles can be formed by directlydepositing multinary metal particulates by, for example, slurryspraying. Conversely, layers of multinary oxide particles can bedeposited and subsequently reduced by, for example, annealing in areducing environment (such as a hydrogen atmosphere) to form multinary,metal particulate layers.

As with precursors comprising multi-phase, particulate materialscomprising metal oxide phases, additional advantages accrue withmixtures of particles and with composites with other precursormaterials. For example, In-rich, Cu—In particles mixed with Cu and/or Separticles, and layers of Cu—In particles combined with layers of Se,facilitate Cu_(x)Se flu_(x)ing and/or simple solid state reactions.

The advantages of this fifth aspect of the invention are realizable on awide variety of materials. For example, just as Cu—In multinary, metalparticles are advantageous for forming CuInSe₂, so, too, In—Sn, Zn—Cdand Zn—Al particles would be advantageous for forming ITO,Zn_(x),Cd_(y)Te, and Al-doped ZnO, respectively. Similarly, just asIn—Cu,₁₁In₉ particles will be advantageous for forming CuInSe₂ ,comparable, multi-phase, mixed-metal composites will be advantageous forother materials. Just as Cu—In particles admixed with Se particles orovercoated with Se will be advantageous for forming CuInSe₂, Zn—Cdparticles admixed with Te particles or overcoated with Te will beadvantageous for forming Zn_(x)Cd_(y)S.

A sixth aspect of the invention is the formation of films from one ormore porous precursor layers intercalated with one or more materials. Byporous we mean a layer containing void space, such as a layer ofparticles. By intercalated we mean that the intercalating materialpartially or substantially fills the voids in the porous layer, such asbetween the individual particles of a particulate layer. Intercalationis distinct from and distinctly advantageous relative to overcoating, asovercoating generally does not fill the voids with the overcoatedmaterial. For example, a Cu—In precursor layer can be overcoated with Seby screen printing, without the Se penetrating into the voids, if any,in the Cu—In precursor layer. This phenomenon is well-known and is usedto great advantage in screen printing electrodes on porous and/orpinhole-ridden films where a penetrating overcoating would electricallyshort-circuit the film. Intercalation purposely seeks to fill the voids,partially or substantially. Another advantage of intercalation is thatit facilitates densification and cohesion of films formed fromparticle-based precursor layers.

Referring to FIG. 3, this aspect of the invention comprises depositingone or more precursor layers 12 of one or more porous precursormaterials 11 on substrate 9, intercalating the precursor layers with oneor more materials 13, and converting said precursor layers to desiredfilms 14.

Intercalated porous precursor layers offer significant advantages oversolid precursor layers and unintercalated porous precursor layers. Solidprecursor layers, such as Cu—In or layered Cu/In, can be overcoated withreactants, such as Se, to facilitate solid state reactions attemperatures lower than those typically required for vapor-phase orgas-phase reactions. Intercalated particulate precursor layers canprovide similar advantages while also providing easier layer depositionand suppressing phase segregation. Unintercalated porous precursorlayers can provide the advantages detailed above in the discussion ofthe second, third, fourth and fifth aspects of the invention;intercalated porous precursor layers provide additional advantages,including low-temperature conversion and better densification.

Porous precursor layers can be intercalated with one or more materialsthat provide advantageous processing pathways and/or that provideconstituents necessary to form the desired final film composition. Thefinal desired film may be formed by reaction solely between the porousprecursor layer and the intercalating materials, or between theintercalated precursor and additional reactants. For example, an oxideprecursor layer, such as Cu₂In₂O₅, can be intercalated with a metalchalcogen, such as Se, to facilitate a solid state reaction and form achalcogenide, such as CuInSe₂. As another example, In-rich Cu₂In₂O₅ canbe intercalated with a copper-containing material, such as Cu, Cu₂O, orCu₂Se, to facilitate Cu_(x)Se fluxing during conversion to CuInSe₂ byreaction with Se vapor. As another example, In₂O3 can be intercalatedwith tin or tin oxide, or, conversely, SnO₂ can be intercalated withindium or indium oxide, to facilitate the formation of an indium tinoxide transparent conductor film. Similarly, a particulate layer ofZnO—CdO can be intercalated with Cd, CdO or CdS to facilitate formationof Zn_(x)Cd_(1−x)S.

Porous precursor layers can be intercalated at any time during or afterthe deposition of the layers. For example, an In-rich Cu₂In₂O₅particulate layer can be intercalated with Cu_(x)O during deposition ofthe particulate layer, by hybrid spraying processes in which coppersalts are dissolved in an aqueous slurry containing In-rich Cu₂In₂O₅,such that spraying onto a heated substrates, in an oxidizingenvironments simultaneously deposits a layer of Cu₂In₂O₅ intercalatedwith Cu_(x)O. As another example, In-rich Cu₂In₂O₅ can be intercalatedwith Cu after deposition of the particulate layer, by solutiondeposition of Cu into the Cu₂In₂O₅ layer.

A film can be formed by interdiffusing the constituents of intercalatedporous layers or a mixture of intercalated and unintercalated layers.For example, CuInSe₂ can be formed by annealing Cu₂In₂O₅ intercalatedwith Se in a reducing atmosphere so as to cause an ion exchange reactionbetween oxygen and selenium. Alternatively, intercalated layers can bereacted with additional reactants. For example, In-rich Cu₂In₂O₅particulate layers intercalated with Cu can be reacted with Se vapor oran overcoated Se layer to form CuInSe₂, and a layer of In-rich Cu₂In₂O₅particles admixed with Se particles and intercalated with Cu can beinterdiffused to form CuInSe₂.

The advantages of this sixth aspect of the invention are realizable on awide variety of materials. For example, just as In-rich Cu₂In₂O₅particulate layers intercalated with Cu can be reacted with Se vapor toform CuInSe₂, Zn-rich ZnO—CdO particulate layers intercalated with Cdcan be reacted with Te vapor to form ZnCdTe₂.

Certain features of this invention apply to all of the aspects andembodiments described herein. For example, suitable particulateprecursor materials as described in the preferred embodiments of thisinvention can vary in size and shape with precursor form, function andcomposition. Particulate precursors intended for bulk materialspreparation can be of any size, preferably of 1 mm diameter or less,more preferably of 0.1 mm or less. Particulate precursors intended forforming 10-100 μm thick films useful for phosphor coatings, acousticsurface wave devices, photodetectors, optical filters,electroluminescent devices and related applications can be of any sizeless than the desired film thickness, preferably of 5 μm diameter orless, more preferably of 1 μm diameter or less. Particulate precursorsintended for forming 0.5-10 μm thick films useful for thin-filmphotovoltaics and related applications can be of any size less than thedesired film thickness, preferably of 1 μm or less, more preferably of0.5 μm or less, most preferably of 0.1 μm or less. Particulate precursormaterials of average size less than the ranges specified here mayprovide equivalent or enhanced results; for example precursor particlesof 0.1 μm or less may be used for bulk materials, thick film and thinfilm applications to advantageous effect.

Particulate precursor materials can be of uniform size, of a broaddistribution of sizes, or of specific narrow distributions of sizes. Forexample, a bulk or thin film material may best be formed by a collectionor layer, respectively, of uniformly-sized precursor particles; a thinfilm material may achieve improved packing density and hence, filmdensity and cohesion, by being formed from a mixture of particulatematerials with a range of sizes; and a thin film material may achieveoptimal film properties by a specific distribution of sizes, such as abimodal distribution of relatively larger, multinary oxide particlesmixed with relatively smaller, metal particles.

Particulate precursor particle shape and form can vary with precursormaterial and preparation method. For example, precursor particles can bespheres, pieces of spheres, plates, irregular solids, and other shapesas may result from various particle formation reactions; and precursorparticles can be solid, hollow, spongy, and other forms as may resultfrom various particle formation reactions. Substantially solid,substantially spherical particles are particularly advantageous forachieving densely-packed precursor layers.

Particulate precursor layers can be deposited by numerous methods,including slurry spraying, spray printing, spin coating, meniscuscoating, slurry painting, slurry casting, screen printing, stencilprinting, writing, nozzle printing, relief printing, intaglio printing,and other such processes as known in the art. Any of these methods isuseful to realize the primary advantages of the particulate materialsprocesses disclosed in the third, fourth, fifth and sixth aspects of theinvention. Slurry spraying and meniscus coating methods provide uniqueadditional advantages for forming high-quality thin films.

Conversion of precursor layers to desired final films as described inthe preferred embodiments of this invention can be effected by numerousmethods, including solid-state, vapor-phase, gas-phase and hybridreactions in, for example, batch or in-line furnaces; liquid-phasereactions in, for example, ion exchange baths; and other such methodsand equipment as known in the art. Thermal annealing in a reducingenvironment is particularly advantageous in providing a simple techniqueto effect ion exchange and interdiffusion reactions to convertparticulate precursors to the desired materials. Temperature and timeprofiles necessary to cause ion exchange and/or interdiffusion vary withthe particular materials being exchanged and/or interdiffused. Typicalgas-solid reactions to form CuInSe₂ include a reaction period atapproximately 150° C. and a high-temperature anneal at approximately400° C. Solid-state reactions occur at lower temperatures, e.g. about50°-75° C. lower than typical gas-solid reactions. Vapor-phase reactionscan occur at low precursor temperatures, provided that a stable sourceof reactant vapor is present, e.g., in a vacuum evaporation system.Reactions with wider-bandgap alloys of CuInSe₂ with Ga and S generallyrequire higher reaction and annealing temperatures, e.g. about 50°-150°C. higher than typical CuInSe₂ reactions. Reactions withoxide-containing precursors generally require higher temperatures thanreactions with metal precursors.

Reactants from Group VB, such as P, As, Sb and Bi, and from Group VIB,such as O, S Se and Te, can be present in solid, liquid, vaporous orgaseous forms. For example, Cu₂O—In₂O₃ can be reacted with Se to formCuInSe₂ , with Se being present as a solid, such as admixed Separticles, or an overcoated solid or particulate Se layer; as a liquid,such as Se-containing ions in solution; as a vapor, such as Se vapor; oras a gas, such as H₂Se.

Conversion of particulate precursors in powder or layer form to desiredfinal bulk or film materials as described in the preferred embodimentsof this invention can be effected by reactions in various environments,including inert atmospheres such as Ar and N₂; reducing atmospheres,such as H₂ and gas mixtures containing H₂; reactive gas atmospheres,such as atmospheres comprising hydride gases such H₂S and H₂Se; reactivevapor atmospheres, such as atmospheres comprising one or more metalvapors such as Se_(vap), S_(vap) and Te_(vap) and such as atmospherescomprising compound vapors such as In_(x)Se; reactive liquids; and othersuch environments as known in the art. Reactive reducing atmospherescomprising a reactive vapor and hydrogen gas are particularlyadvantageous.

Conversion of particulate precursors to desired final bulk materials asdescribed in the preferred embodiments of this invention can be effectedby numerous methods, including vapor-phase and gas-phase reactions in,for example, fluidized bed reactors or calciners; liquid-phase reactionsin, for example, stirred reactors; and other such methods as known inthe art.

Suitable substrates onto which to deposit precursor layers as describedin the preferred embodiments of this invention include any substratetype suitable for the specific application of the final film, includingglass, glass coated with substantially transparent conductors such astin oxide, indium tin oxide and zinc oxide, glass coated with metalssuch as molybdenum, titanium and tantalum, glass coated with conductivecompounds, glass coated with alkali ion diffusion barrier layers, low-Naglasses, ceramics, metal sheets and foils, polymers, and other suchsubstrates known in the art. Particularly useful are substrates likeconductor-coated glass, especially glass coated with molybdenum orconductive metal oxides.

Conversion of precursor layers to desired final films as described inthe preferred embodiments of this invention can be effected by numerousmethods, including solid-state, vapor-phase, gas-phase and hybridreactions in, for example, batch or in-line furnaces; liquid-phasereactions in, for example, ion exchange baths; and other such methodsand equipment as known in the art. Intercalation of particulateprecursor layers as described in the preferred embodiments of thisinvention can be effected by numerous methods, including spray pyrolysisand hybrid slurry/pyrolysis spraying; solution or chemical bathdeposition, electroplating; vapor-phase deposition at precursor layertemperatures below those where substantial reaction between theprecursor layer and the vapor occurs; and other such methods as known inthe art.

The following nonlimiting examples are illustrative of the invention.

EXAMPLE 1

Single-phase, mixed-metal, Cu₂In₂O₅ particulate materials with anaverage particle diameter of about 0.1 μm were prepared by atomizing anaqueous solution containing 0.25 mM each of copper nitrate and indiumnitrate in an ultrasonic nebulizer to form a fine aerosol; transportingthe aerosol into a quartz tube furnace operated at about 900° C., usingoxygen as a carrier gas; and capturing the resulting oxide particulateson a filter membrane at the exit of the furnace.

Particulate average particle diameter is determined by solutionconcentration and aerosol droplet size; for example, particle diametersof about 0.05 to 0.5 μm are formed from ca 5 μm droplets by varying theconcentration of nitrates in the precursor solution from about 0.015 to15 milli-Molar. Particulate Cu/In atomic composition ratio is determinedfor the most part by the relative concentration of nitrates in theprecursor solutions; for example, particle Cu/In ratios of about 0.65 to1.15 result from substantially equivalent Cu/In ratios in precursorsolutions.

EXAMPLE 2

Multi-phase, mixed-metal, Cu₂O—In₂O₃ particulate materials were preparedaccording to Example 1 except that nitrogen was substituted for oxygenas the carrier gas.

EXAMPLE 3

Multi-phase, mixed-metal, Cu—In₂O₃ metallic particulate materials wereprepared according to Example 1 except that 10 vol % H₂ in N₂ wassubstituted for oxygen and the furnace temperature was about 500° C.

Given that metal oxides can be reduced to metals by annealing insuitably reducing atmospheres, it is evident that particles preparedaccording to this Example except that higher concentrations of H₂ aresubstituted for 10 vol % H₂ in N₂ will yield multinary, mixed-metalCu—In particles.

EXAMPLE 4

Single-phase Cu₂In₂O₅ particulate materials with an average particlediameter of about 0.1 μm were prepared according to Example 1.Water-based slurries of about 5 to 10 g/L solids loading were preparedby dispersing particulates in de-ionized water using ultrasoniccavitation. Addition of a few weight percent of ethanol and/or acommercial dispersant such as Rohm and Haas Duramax 3002 aideddispersion for some particulate batches. Precursor layers were depositedby spraying the slurries at 1-2 mL/min onto heated glass substratesusing air or nitrogen as the spraying gas. The resulting Cu₂In₂O₅particulate layers were macroscopically uniform, adhesive and cohesive,and were microscopically uniform with good coverage.

EXAMPLE 5

Multi-phase, mixed-metal, Cu₂O—In₂O₃ particulate materials with anaverage particle diameter of about 0.1 μm were prepared according toExample 2. Precursor layers were spray-deposited using water-basedslurries according to Example 4. Precursor-coated substrates wereannealed in Se vapor at about 425° C. for about one hour in a reducingatmosphere of 7 to 10 vol % H₂ in N₂ to convert the oxide precursorlayers to CuInSe₂ films. The grain size of the CuInSe₂ films wasexamined by electron microscopy and found to be up to 2.5 times largerthan the average particle size of the Cu₂O—In₂O₃ precursors.

Particulate precursor materials with average particle diameters of 0.06to 0.5 μm were investigated and found workable, provided that layerthicknesses were at least a few particle diameters.

Slurry solids loadings of about 0.2 to 40 g/L were investigated andfound workable, provided that the substrate temperature and sprayingrate were adjusted to mitigate liquid pooling on the substrate surfaceand solvent evaporation in-flight.

Precursor layers were also deposited with ethanol-based and propyleneglycol-based slurries in which the water and dispersant additives weresubstituted for denatured ethanol and propylene glycol, respectively.

Comparable precursor layers were also deposited by a casting process inwhich the slurry was applied to the substrates and allowed to air dry.

Comparable results were obtained using bare and tin-oxide-coated glasssubstrates.

Given that metal oxides can be converted to metal chalcogenides byannealing in the presence of one or more chalcogenides, it is evidentthat such particulate precursors will be converted into sulfides,tellurides, phosphides, arsenides, antimonides, other such compounds,and alloys of these by substituting or adding, respectively, suitablesources of the appropriate reactants during the conversion process.

EXAMPLE 6

Single-phase Cu₂In₂O₅ particulate materials with an average particlediameter of about 0.1 μm were prepared according to Example 1. Precursorlayers deposited using water-based slurries were prepared according toExample 4 and annealed according to Example 5, yielding CuInSe₂ filmswith CuInSe₂ grain sizes of at most 1.3 times the average particle sizeof the Cu₂In₂O₅ precursors. The resulting CuInSe₂ films are inferior indensity and cohesion relative to comparably processed multi-phaseCu₂O—In₂O₃ precursors.

EXAMPLE 7

Multi-phase, mixed-metal, Cu—In₂O₃ particulate materials with an averageparticle diameter of about 0.1 μm were prepared according to Example 3.Water-based slurries were prepared according to Example 4, and precursorlayers were deposited on Mo-coated glass substrates according to Example5. The precursor-coated substrates were annealed according to Example 4.The grain size of the CuInSe₂ films were examined by electron microscopyand found to be about 2 to 5 times larger than the average particlesizes of the Cu—In₂O₃ precursors.

Given that multi-phase Cu—In₂O₃ particulate materials yieldsignificantly larger CuInSe₂ film grain sizes than single-phase Cu₂In₂O₅and multi-phase Cu₂O—In₂O₃ particulates, and that multi-phase Cu—Insolid layers are reported to yield large CuInSe₂ grains, it is evidentthat multinary metallic Cu—In particulates prepared according to Example3 will yield large CuInSe₂ film grains.

EXAMPLE 8

Single-phase Cu₂In_(1.5)GaO_(0.5)O₅ particulate materials with anaverage particle diameter of about 0.1 μm were prepared according toExample 1, except gallium nitrate was substituted for a portion of theindium nitrate. Water-based slurries of Cu₂In_(1.5)Ga_(0.5)O₅particulate materials were prepared according to Example 4. Precursorlayer deposition was carried out according to Example 4, and annealingwas carried out according to Example 5, yielding CuIn_(0.75)Ga_(0.25)Se₂films. Other desirable constituents, such as alloying metals—for exampleAl and Ag—in the CuInSe₂ alloy materials family, and dopant and additivematerials—for example Na, Cd, Zn, P and As—in the CuInSe₂ alloymaterials family, can be incorporated in the desired concentration in alike manner.

Given that alloy constituents, dopants and additive materials can beincorporated into particulate materials by adding sources of therespective alloy constituents, dopants and additive materials to thesource solutions to be atomized, it is evident that multi-phase,mixed-metal particles comprising multiple metal oxide phases or both ametal oxide and a non-oxide phase can be prepared with various alloyconstituents, dopants and additive materials by adding the appropriatesources of the alloy constituents, dopants and/or additive materials tosource solutions prepared and processed according to Examples 2 and 3,respectively. Likewise, it is evident that multinary metallic particleswith various alloy constituents, dopants and additive materials can beprepared by appropriately adding to the solutions processed according toExample 3.

EXAMPLE 9

Water-based slurries of about 0.5 μm Cu_(x)O particulate materials wherex˜1 were prepared according to Example 4, and dilute copper nitratesolutions were added to the slurries. Precursor layers of Cu_(x)Ointercalated with Cu_(y)O where y˜1 were deposited by spraying theseslurry solutions onto heated substrates so as to deposit particles byslurry spraying, and simultaneously by co-depositing Cu_(y)O by spraypyrolysis so as to partially intercalate the particles. Hybridslurry/pyrolysis spray deposition of a comparable solution in a suitablyreducing atmosphere will yield oxide particles intercalated with Cumetal.

EXAMPLE 10

Water-based slurries of about 0.25 μm Cu₂In₂O₅ particulate materialswere prepared according to Example 4, and dilute copper nitratesolutions were added to the slurries. Precursor layers of Cu₂In₂O₅intercalated with Cu_(x)O where x˜1 were deposited by spraying theseslurry solutions onto heated substrates so as to deposit particles byslurry spraying, and simultaneously by co-depositing Cu_(x)O by spraypyrolysis so as to partially intercalate the particles.

Although this invention is described with respect to a set of preferredaspects and embodiments, modifications thereto will be apparent to thoseskilled in the art. Therefore, the scope of the invention is to bedetermined by reference to the claims which follow. Throughout the textand the claims, use of the word “about” in relation to a range ofnumbers is intended to modify both the low and the high values stated.

What is claimed is:
 1. A process for making a photovoltaic devicecomprising a film structure comprising a mixed-metal compound material,comprising: depositing at near atmospheric pressure a layer of aprecursor material comprising multi-phase, mixed-metal particlescomprising a metal oxide phase, and reacting the precursor material withat least one reactant material, to form the compound material at atemperature substantially below the melting point of the compoundmaterial.
 2. A process according to claim 1, wherein the multi-phase,mixed-metal particles comprise multiple metal oxide phases.
 3. A processaccording to claim 1, wherein the multi-phase, mixed-metal particlescomprise at least one phase substantially enveloping at least one otherphase.
 4. A process according to claim 1, wherein the precursor materialcomprises multi-phase particulate materials and other particulatematerials.
 5. A process according to claim 1, wherein the precursormaterial is deposited as one or more layers on a substrate.
 6. A processaccording to claim 1, wherein the reactant materials are present asparticles admixed with the precursor material or as layers overcoated onto the precursor material.
 7. A process according to claim 1, whereinthe mixed-metal compound material is a Group VB or VIB compound materialand wherein at least one of the reactant materials comprises one or moreGroup VB or VIB elements.
 8. A process according to claim 1, wherein theparticulate material comprises one or more elements from Groups IBand/or IIIB.
 9. A process for making a photovoltaic device comprising afilm structure comprising a mixed-metal compound material, comprising:depositing at near atmospheric pressure a layer of a precursor materialcomprising multi-phase, mixed-metal particles comprising a metal oxidephase and a non-oxide phase, and reacting the precursor material with atleast one reactant material, to form the compound material at atemperature substantially below the melting point of the compoundmaterial.
 10. A process according to claim 9, wherein the non-oxidephase comprises a metal phase.
 11. A process according to claim 9,wherein the non-oxide phase is a non-oxide chalcogenide compound.
 12. Aprocess according to claim 9, wherein the multi-phase, mixed-metalparticles comprise at least one phase substantially coating at least oneother phase.
 13. A process according to claim 9, wherein the precursormaterial comprises other particulate materials in addition to themulti-phase, mixed-metal, particulate materials.
 14. A process accordingto claim 9, wherein the precursor material is deposited as one or morelayers on a substrate.
 15. A process according to claim 9 wherein thereactant materials are present as particles admixed with the precursormaterial or as layers overcoated on the precursor material.
 16. Aprocess according to claim 9, wherein the mixed-metal compound materialis a Group VB or VIB compound material and wherein at least one of thereactant materials comprises one or more Group VB or VIB elements.
 17. Aprocess according to claim 9, wherein the particles comprise one or moreelements from Groups IB and/or IIIB.
 18. A process for making a film ofa compound material useful for fabricating photovoltaic devices,transparent conductor film structures, or other film structures,comprising: intercalating at near atmospheric pressure one or moreporous, mixed-metal precursor layers with one or more intercalatingmaterials; and heating the layers and intercalating materials at atemperature substantially below the melting point of the compoundmaterial for a time sufficient to cause interdiffusion of the layers andintercalating materials, thereby forming a film the composition of whichis different from the precursor layers.
 19. A process according to claim18, wherein the porous precursor layers comprise a metal oxide, and theintercalating materials comprise a metal.
 20. A process according toclaim 18, wherein the porous precursor layers comprise a metal oxide,and the intercalating materials comprise a metal oxide.
 21. A processaccording to claim 18, wherein the porous precursor layers comprise ametal, and the intercalating materials comprise a metal.
 22. A processaccording to claim 18, wherein the porous precursor layers comprise ametal oxide, and the intercalating materials comprise a non-oxidecompound.
 23. A process according to claim 18, wherein theinterdiffusion proceeds by a solid-state reaction of one or more of theporous precursor layers and the intercalating materials.
 24. A processaccording to claim 18, wherein film formation proceeds through agas-phase, vapor-phase, liquid-phase, and/or solid-state reaction withone or more reactants in addition to the porous precursor layers andintercalating materials.
 25. A process according to claim 18, whereinfilm formation proceeds through a solid-state reaction between theprecursor layers, the intercalating materials, and one or more reactantlayers overcoated on to said precursor layers and intercalatingmaterials.
 26. A method of providing a film structure of a compoundmaterial useful for fabricating photovoltaic devices, transparentconductor film structures, or other electronic structures, comprising:preparing at near atmospheric pressure a layer of mixed-metal sub-micronparticles intercalated with a penetrating ion-exchange material; andheating said layer at a temperature substantially below the meltingpoint of the compound material to form the film structure thecomposition of which is different from the particles.
 27. A methodaccording to claim 26, wherein the mixed-metal particles comprise indiumand wherein the intercalating material comprises copper.
 28. A methodaccording to claim 26, wherein the mixed-metal particles comprise copperand indium and wherein the intercalating material comprises selenium.